1. Field of the Invention
The invention relates generally to quenching furnace effluent. More particularly, the invention relates to a process and apparatus for quenching effluent cracked in a furnace using inert solid particles to provide the reaction heat. The invention is particularly adapted for embodiment in an apparatus and process for use in the Thermal Regenerative Cracking (TRC) process, as described in U.S. Pat. No. 4,061,562 to McKinney et al and U.S. Pat. No. 4,097,363 to McKinney et al.
2. Description of the Prior Art
In the technology of thermally cracking hydrocarbon feedstocks to produce olefins, it has long been necessary to stop the reaction of the cracked effluent by rapidly cooling the effluent. Various techniques and apparatus have been provided to facilitate quenching. Both direct quench, wherein a fluid material is introduced directly into the effluent stream, and indirect quench, wherein, heat exchange is performed by heat transfer through the walls separating the hot and cold side of the exchanger are commonly employed. An illustration of the indirect heat exchanger is shown in U.S. Pat. No. 3,583,476 (Herman N. Woebcke et al).
Process and equipment have been recently developed to crack hydrocarbons in tubular reactors employing solid-gas contact. The solids are essentially inert particulate materials which are heated to high temperatures and intimately mixed with the hydrocarbon feedstock to provide the heat necessary to crack the hydrocarbon.
The existing solid-gas contact processes employing inert particulate solids to provide the heat necessary for reaction typically separate the particulate solids from the gas before quench occurs. An illustration is seen in U.S. patent application Ser. No. 055,148 filed July 6, 1979 (Gartside et al).
However, in the noncatalytic temperature dependent endothermic reaction processes, systems have been developed to quench the entire products stream after the requisite reaction period. The problem is that with heavy feedstocks at high severities, short residence times are desirable. The combination of heavy feedstock, high severity and short residence times impose severe operating problems on the heat recovery devices.
In connection with a current TRC installation, a 90% separation occurs in the primary separator. This is followed by an oil quench to 1300.degree. F., and a cyclone to remove the remainder of the solids. The mix is then quenched again with liquid to 600.degree. F. Thus, all the available heat from the reaction outlet temperature to 600.degree. F. is rejected to a circulating oil stream. Steam is generated from this oil at 600 psig, 500.degree. F. The subject invention is used to avoid exchanger fouling when cracking heavy feeds at low steam dilutions and high severities in the TRC. However, instead of an oil quench, a circulating solids stream could be used to quench the effluent. As in the reaction itself, the coke would be deposited preferentially on the solids thus avoiding fouling. These solids can be held at 800.degree. F. or above, thus permitting the generation of high pressure steam (1500 psig+) which increases the overall thermal efficiency of the process. The oil loop can not operate at these temperatures due to instabilities (too many light fractions are boiled off, yielding an oil that is too viscous).